In testing new pecan cultivars, its important to evaluate each clone over several years and at several locations. This year provided some good examples of how location can effect cultivar performance.
We are currently testing two early ripening clones developed by the USDA pecan breeding project at several locations. The photo at right shows nut samples of USDA 75-8-5 collected from two locations--Chetopa, KS and New Madrid, MO. At the Chetopa site the trees are growing in a heavy clay soil without irrigation while in New Madrid the trees are growing in an irrigated sandy soil. Both locations suffered extremely hot and dry summers but you can definitely see the benefits of irrigation on nut size and shape. 
Last summer, soil moisture availability had an overwhelming impact of nut size and shape. However, during years of normal rainfall we would still see differences in nut size between these two locations. Trees growing in heavy clay soils tend to produce smaller pecans than trees of the very same cultivar grown in loamy or sandy loam soil. But, there is one advantage of growing pecans in a heavy clay soil--Clay soils have a greater soil moisture holding capacity. During drought years, quality kernels can be produced without irrigation in a clay soil while kernels produced by trees growing on sandier, non-irrigated soils will be shriveled.